Can you confirm that this kind of "tap" is generally done for solar backfeed applications? It sounds to me like the 8kw you refer to is due to not being allowed to backfeed a traditional 200a panel with more than 20% of the overall rating. So 40 amp breaker would be the max for solar backfeed, and then my guess is a further derate to 80% of that due to it being considered a continuous load, so 32a * 240 = 7,680 watts.
I don't see what the benefit for doing this kind of install would be unless it was because the main panel was only 100a rated, but the meter and meter base and the service drop was rated for 200a and so you would be creating a second "service disconnect" with its own "panel" just to serve the car charging?
This would totally work, but I wonder how often you find a scenario where the service drop and meter base is already rated for over what the inside panel are rated for? (perhaps this is really common since I don't think they really make meters below a 200a rating anymore I don't think?)
I do totally agree that the labor involved in swapping a service panel out is a lot! Often the wires are not all of sufficient length and they need to be extended, etc... Though often these service upgrades are a really good thing since they replace old and questionable ancient panels with new and modern ones that are easier to get parts for, etc....